1. Field of the Invention
This invention relates to edible sugars. More particularly, it relates to fructose obtained by the isomerization of dextrose. Of specific relevance is a process for the concurrent production of anhydrous crystalline fructose and a syrup consisting essentially of fructose and dextrose.
Also of specific relevance are a process of crystallizing fructose by cooling a solution of fructose such that differing levels of supersaturation are produced during different periods of crystal growth and a process for producing a purified and concentrated fructose syrup.
2. Background of the Invention
LIQUID-PHASE FRUCTOSE PRODUCTS
Fructose is a monosaccharide highly valued as a nutritive sweetener. The vast majority of fructose sold in this country is derived from corn starch with the principal form of the product being High Fructose Corn Syrup (HFCS). The syrups of commerce range from 42% to 90% by weight fructose on a dry solids basis. (As used hereinafter, including the claims, "dsb" shall mean "by weight on a dry solids basis".) The remainder is predominately dextrose. The HFCS commonly used as a sucrose replacement in soft drinks typically comprises 55% fructose, 41% dextrose, and 4% higher saccharides (all percentages dsb). The solids content of such a syrup is usually about 77% by weight.
On an industrial scale, the production of HFCS commences with the enzymatic liquefaction of a purified starch slurry. The principal source of raw material in the United States is corn starch obtained by the wet milling process. However, starches of comparable purity from other sources can be employed.
In the first step of a typical process a starch slurry is gelatinized by cooking at high temperature. The gelatinized starch is then liquefied and dextrinized by thermostable alpha-amylase in a continuous two-stage reaction. The product of this reaction is a soluble dextrin hydrolysate with a dextrose equivalent (DE) of 6-15, suitable for the subsequent saccharification step.
Following liquefaction-dextrinization, the pH and temperature of the 10-15 DE hydrolysate is adjusted for the saccharification step. During saccharification the hydrolysate is further hydrolyzed to dextrose by the enzymatic action of glucoamylase. Although saccharification can be carried out as a batch reaction, a continuous saccharification is practiced in most modern plants. In the continuous saccharification reaction, glucoamylase is added to a 10-15 DE hydrolysate following pH and temperature adjustment. The carbohydrate composition of typical high-dextrose saccharification liquor is: 94-96% dextrose, 2-3% maltose; 0.3-0.5 maltotriose; and 1-2% higher saccharides (all percentages dsb). The product will typically be 25 to 37% dry substance. This high-dextrose hydrolysate is then refined to produce dextrose feedstock for the isomerization reaction.
Preparation of very high-quality dextrose feedstock for isomerization is necessary because of the very low color and ash specifications of the finished HFCS. A high-purity feedstock is also required for efficient utilization of the immobilized isomerase enzyme column.
Immobilized isomerase enzyme columns are used continuously over a period of several months. During this period very large volumes of dextrose feedstock pass through the columns. Extremely low levels of impurities such as ash, metal ions, and/or protein in the feedstock can accumulate and lead to decreased productivity of the enzyme. For these reasons dextrose feedstock is refined to a color of 0.1 (CRA.times.100) and a conductivity of 5-10 micromhos.
Carbon-treated, filtered, and deionized, high-dextrose liquor is evaporated to the proper solids level for isomerization. In addition, the feedstock is chemically treated by the addition of magnesium ions, which not only activate the immobilized isomerase, but also competitively inhibit the action of any residual calciums ions, which are potent inhibitors of isomerase.
The isomerization reaction, which converts some of the dextrose to fructose, is commonly carried out on a stream comprising 94-96% (dsb) dextrose and 4-6% (dsb) higher saccharides, at 40-50% dry substance. The stream has a pH of 7.5-8.2 at 25.degree. C. and will be subjected to the action of the isomerase enzyme for 1/2 to 4 hours at 55.degree.-65.degree. C.
The conversion of glucose to fructose is a reversible reaction with an equilibrium constant of about 1.0.degree. at 60.degree. C. Thus, one would expect to obtain a fructose level of about 47-48% at equilibrium, starting from a feedstock continuing 94-96% dextrose. However, the reaction rate of the isomerization near the equilibrium point is so slow that it is prudent to terminate the reaction at a conversion level of about 42% fructose to achieve practical reactor residence times.
In a given isocolumn (immobilized isomerase column), the rate of conversion of dextrose (glucose) to fructose is proportional to the enzyme activity of the immobilized isomerase. This activity decays over time in a nearly exponential fashion. When the column is new and the activity is high, the flow of feedstock through the column is relatively high, since a shorter residence time is required to achieve the 42% fructose level. As the usage life of the column increases, the flow through the column must be reduced proportionately to provide a longer residence time, compensating for the lowered activity in order to achieve a constant conversion level.
In practice, parallel operation of multiple isocolumns is used to minimize production fluctuations with respect to capacity and conversion level. In this arrangement each isocolumn can be operated essentially independently of the others. The variation in total flow of the isocolumns must be maintained within relatively narrow limits because of the requirements of evaporation and other finishing operations. In practice, flow cannot be precisely controlled at all times so as to obtain a 42% fructose stream, but this can easily be achieved on an average basis.
One of the most critical operating variables in such a process is the internal isocolumn pH. The operating pH is usually a compromise between the pH of maximum activity (typically around pH 8) and the pH of maximum stability (typically pH 7.0-7.5). This is complicated by the fact that the dextrose feedstock is not pH stable at temperatures around 60.degree. C. Some decomposition occurs producing acidic by-products which results in a pH drop across the isocolumn during operation.
Following isomerization, the typical manufacturing process employs secondary refining or polishing of the 42% HFCS product. Some additional color is picked up during the chemical treatment and isomerization when the feedstock is held at a higher pH and temperature for a period of time. The product also contains some additional ash from the chemicals added for isomerization. This color and ash are removed by secondary carbon and ion exchange systems. The refined 42% HFCS is then typically evaporated to 71% solids for shipment.
The use of activated carbon to purify sugar syrups is generally known. U.S. Pat. No. 1,979,781 (van Sherpenberg) discloses mixing a raw sugar syrup (i.e., one not mixed with glucose syrup or with invert sugar syrup) at 60.degree. Brix (60% dry solids) with 1 to 2% by weight activated carbon and heating to 134.degree. C. for a short period of time. U.S. Pat. No. 2,763,580 (Zabor) broadly discloses treatment of sugar liquors (e.g., cane, beet or corn sugars) having solids contents of between 10 and 60%, especially 20 to 56%, by weight at 125 to 200.degree. F. with activated carbon. The patent discloses that partial treatment can be carried out at one concentration or condition, after which the treatment can be completed at a higher concentration (obtained by evaporation) or other condition.
Various patents directed to the production of corn syrups containing fructose incidentally disclose carbon-treatment and subsequent concentration of aqueous solutions having varying fructose concentrations (dsb) and varying levels of dry solids. U.S. Pat. Nos. 3,383,245 (Scallet et al.) and 3,690,948 (Katz et al.) disclose carbon-treating fructose containing syrups having about 20% (dsb) fructose at about 40% dry solids and subsequently concentrating the syrups (e.g., by evaporation to 70-83% dry solids).
U.S. Pat. No. 3,684,574 (Katz et al.) discloses carbon-treatment of a syrup containing about 20% (dsb) fructose at a dry solids as low as 20% dry solids and subsequent concentration of the syrup. U.S. Pat. No. 4,395,292 (Katz et al.) discloses feeding a carbon-treated mixture of fructose and dextrose having from 10 to 70% dry solids, preferably 40%, to a fractionating column and concentrating the fructose containing extracts. The '292 patent discloses that extracts containing over 90% fructose can be obtained and discloses an example (Example No. 7) wherein a 40% dry solids feed was fractionated to produce a fraction having 100% (dsb) fructose at 9% dry solids.
The HFCS product from the isomerization reaction typically contains 42% fructose, 52% unconverted dextrose, and about 6% oligosaccharides. For reasons previously discussed, this product represents the practical maximum level of fructose attainable by isomerization. In order to obtain products with higher levels of fructose, it is necessary to selectively concentrate the fructose. Many common separation techniques are not applicable for this purpose, since they do not readily discriminate between two isomers of essentially the same molecular size. However, fructose preferentially forms a complex with different cations, such as calcium. This difference has been exploited to develop commercial separation processes.
There are basically two different commercial processes currently available for the large-scale purification of fructose. In both instances, resins in the preferred cationic form are used in packed bed systems. One process employs an inorganic resin leading to a selective molecular absorption of fructose (see, R. J. Jensen, "The Sarex Process for the Fractionation of High Fructose Corn Syrup," Abstracts of the Institute of Chemical Engineers, 85th National Meeting, Philadelphia, Pa., 1978).
Chromatographic fractionation using organic resins is the basis for the second commercial separation process (see, K. Venkatasubramanian, "Integration of Large Scale Production and Purification of Biomolecules," Enzyme Engineering, 6:37-43, 1982). When an aqueous solution of dextrose and fructose (e.g., 42% HFCS) is fed to a fractionating column, fructose is retained by the resin to a greater degree than dextrose. Deionized and deoxygenated water is used as the eluent. Typically, the separation is achieved in a column packed with a bed of low crosslinked, fine-mesh, polystyrene sulfonate cation exchange resin using calcium as the preferred salt form. The enriched product which contains approximately 90% fructose is referred to as Very Enriched Fructose Corn Syrup (VEFCS). This VEFCS fraction can be blended with the 42% HFCS feed material to obtain products having a fructose content between 42% and 90%. The most typical of these products is 55% Enriched Fructose Corn Syrup, which is sometimes referred to as EFCS or 55 EFCS. U.S. Pat. No. 4,395,292 (Katz et al.) discloses an example (Example No. 1) of fractionating a mixture of fructose and dextrose into various fractions and combining fructose-enriched fractions to produce a syrup containing 55.8% (dsb) fructose. This same example also discloses single fractions having high concentrations (dsb) of fructose (e.g., 75.1% (dsb)) and discloses combining fractions containing lesser concentrations of fructose (e.g., 64.5% (dsb) with 58.2% (dsb) fructose).
The treatment of other raffinate streams in the fractionation process is an important consideration. In general, the dextrose-rich raffinate stream is recycled to the dextrose feed of the isocolumn system for further conversion to 42% HFCS. A raffinate stream containing dextrose and fructose and having a fructose level higher than that of the feed stream can be recycled through a fractionator to maintain a high solids level and to reduce water usage. A raffinate stream rich in oligosaccharides can be recycled to the saccharification system.
Since water is used as the elution media, it has a great impact on the overall evaporation load on the system. Very low solids concentrations increase the risk of microbial contamination within the system. Thus, the most important design parameter dictating overall process economics is the maximization of solids yield at acceptable purity while minimizing the dilution effect of the eluant rinse. The efficiency of feed and water usage must be maximized for optimal yield. The yield is important to reduce the cost of reisomerization.
Procedures available for achieving these goals include recycling techniques, higher equalization of the resin phase with proper redistribution in a packed column, and the addition of multiple entry and exit points in the column. These approaches can be used to increase the purity and the yield.
In a batch fractionation system, a small apparent increase in the purity of feed to the fractionating column, that is, higher fructose levels, results in a much larger gain in production through increased yield at a given product purity. In practice, this translates into maximization of the ratio of the sugar volume fed per volume of resin per cycle, minimization of the ratio of the water column required per volume of resin per cycle, and careful fluid distribution to the columns.